The present invention relates to hydromechanical transmissions, and more particularly to an improvement of a hydromechanical transmission of the type wherein low and high speed drive power trains are selectively completed in response to variation of the speed ratio between an input shaft and an output shaft.
In conventional hydromechanical transmissions of this kind, the speed ratio of the output shaft against the input shaft is controlled in accordance with variation of the displacement ratio of an adjustable positive displacement pump-motor and the low speed drive power train is switched over to and from the high speed drive power train when the speed ratio reaches a predetermined value. In the pump-motor, however, it is very difficult to eliminate fluid leakage between a cylinder barrel and a valve plate assembled within the pump-motor. Thus, due to the fluid leakage of the pump-motor, the actual speed ratio conducted under control of the pump-motor differs inevitably from the speed ratio figured out in the designing stage. As a result, the switchover between the low and high speed drive power trains may not be surely conducted.
To solve the above-mentioned drawback, in a copending U.S. patent application Ser. No. 765,532 filed on Feb. 1, 1977, the inventors have been proposed a hydromechanical transmission wherein speed ratio detecting means is provided to detect lower and higher limits of a predetermined speed ratio and clutch control means is provided to selectively operate the low and high speed range clutches in a range defined by the lower and higher limits of the predetermined speed ratio so as to surely conduct the switchover between the low and high speed drive power trains. In the hydromechanical transmission mentioned above, it is, however, experienced that the low and high range clutches are frequently engaged and disengaged when the actual speed ratio reaches in the predetermined range described above. This causes unpleasant drive feeling and unnecessary wearing of the clutches.